1. Field of the Invention
The present disclosure generally relates to methods of controlling SMA actuators, and more particularly, to a method of controlling an SMA actuator utilizing resistance change (i.e., the change in resistance inherent to the actuator over an actuation cycle).
2. Discussion of Prior Art
Shape memory alloy (SMA) actuators, in the Martensitic state, are activated by heating the SMA material to a temperature that is above a prescribed value. This causes the material to undergo phase transformation from a Martensitic to an Austenitic state, wherein it contracts and in the process is used to do work. SMA wires, for example, are resistively heated by applying an electrical current there through. Concerns with this approach include overheating, i.e., applying an excess of heat energy above what is required to actuate the wire, and overloading, i.e., applying a stress load, for example, by blocking the work, that is greater than the recommended tensile load of the SMA wire. More specifically, it is appreciated that overheating and overloading can cause longer cooling times, reduced system response bandwidth, and in some cases damage to the wire. It is therefore desirable to have an effective and robust means of controlling wire actuation to prevent overheating and overloading, and to provide consistent output and streamlined actuation over the life of the actuator.
Traditionally, various external sensors and/or mechanical devices, such as temperature and position sensors, have been used to alleviate concerns relating to overheating, overloading, and variation/degradation in output. However, these provisions add to the complexity, costs, and packaging requirements of conventional actuators. Closed-loop controls have been developed that monitor absolute actuator resistance as feedback, however, these methods often present inaccuracy concerns due to susceptibility to noise, and variations in ambient conditions.